Electromagnetic flow meters use the principle of induction in electrodynamics for volumetric flow measurements and are known from a multitude of publications and/or disclosures. Charge carriers of the medium, which move orthogonally to an applied magnetic field, induce a measurement voltage in measurement electrodes that are positioned essentially orthogonal to the direction of the flow of the medium and orthogonal to the direction of the magnetic field. The measurement voltage induced in the measurement electrodes is proportional to the average flow velocity of the medium through a cross section of the measurement tube, i.e. proportional to the volumetric flow. If the density of the medium is known, the mass flow in a pipe and measurement tube, respectively, can be determined. The measurement voltage is usually picked up via a pair of measurement electrodes, which are positioned, in reference to the coordinates along the measurement tube axis, in the region of the maximum magnetic field strength and where, it follows, the maximum measurement voltage is to be expected. Usually, the electrodes are in electrical contact with the medium; however, electromagnetic flow meters having contactless capacitively coupled electrodes are also known.
The measurement tube can thereby be manufactured from either nonmagnetic electrically conducting material, e.g. stainless steel, or consist of an electrically isolating material. If the measurement tube is manufactured from an electrically conducting material, then in the region where it is in contact with the medium it is lined with an electrically isolating material. The liner, depending on temperature and medium, consists, by way of example, of a thermoplastic, duroplastic or a plastic elastomer. There are, however, electromagnetic flow meters known with a ceramic liner.
An electrode can be essentially divided into an electrode head, which at least partially comes into contact with the medium that flows through the measurement tube, and an electrode shaft, which is almost wholly inserted into the wall of the measurement tube.
Along with the magnetic system, the electrodes are the central components of an electromagnetic flow meter. In the embodiment and arrangement of the electrodes, one should pay attention that they are mounted on the measurement tube as simply as possible and that subsequently during measurement operation, no leakage problems arise; even further, the electrodes should feature a sensitive and at the same time reliable measurement signal registration.
Along with the measurement electrodes, which serve to pick up a measurement signal, additional electrodes in the form of reference or grounding electrodes are often built into the measurement tube, which serve to measure an electric reference potential or to recognize a partially filled measurement tube.
The electrodes mostly comprise an elongated electrode shaft, with diversely formed indentations. The WO 2009/071615 A2 discloses an electrode with an electrode shaft with many truncated conically formed sealing lips, wherein the diameters of the lips vary in their magnitudes. Equally important are electrodes that have saw tooth formed indentations, wherein the diameter of the electrode at the tips of the teeth of the saw tooth formed indentations varies in its magnitude.
In DE 10 2008 038 161 and DE 10 2008 038 162, electrodes along with other components of an electromagnetic flow meter are fabricated in a common plastic molding process and separate from the fabrication of the measurement tube and process connections, respectively.
WO 10015534 A1 shows an electromagnetic flow meter with a sensor, wherein the sensor comprises a measurement tube, through which a medium can flow, comprises a magnetic system for producing a magnetic field, comprises at least one electrode, which is inserted into the measurement tube, for registering an electrical voltage, wherein a fixing means is provided, which serves to fix the electrode onto the measurement tube, wherein the fixing means is composed at least partially out of a plastic, and wherein the fixing means is joined to the measurement tube by plastic welding, as well as a process for the fabrication of an electromagnetic flow meter, wherein the electromagnetic flow meter comprises a measurement tube, wherein the measurement tube comprises a boring, wherein an electrode is inserted into the boring, wherein the electrode is fixed to the measurement tube by means of plastic welding. Thereby, in an embodiment, the electrodes comprise at least one sealing element, which seals the measurement tube in the region of the boring, wherein the sealing element can be partially inserted into a recess of the electrode shaft. However, there is a danger of damaging the sealing element during the mounting of the electrode in a boring in the measurement tube.